how can I change my security questions?

Change your security questions

1. Sign in to appleid.apple.com.
2. In the Sign-In and Security section, select Account Security.
3. Under Security questions, select Change security questions. 
4. Choose your new security questions and their answers, then select Update.
5. Enter your Apple ID password, then select Done.

It's very important to remember the answers to your security questions to avoid being locked out of your account. And make sure that you add and verify your rescue email address. If you forget the answers to your security questions, you'll need a rescue email address to help you reset them. Learn what to do if you forget the answers to your security questions.

See:

Change your Apple ID security questions - Apple Support